Gyllenstierna was born in Stockholm, Sweden, the son of major general, friherre, Göran Gyllenstierna and his wife Anna. He was commissioned as an officer in the Life Regiment of Horse with the rank of fänrik in 1932. He represented Sweden in modern pentathlon in Hungary and Germany 1935-1936 and competed at the 1936 Summer Olympics. Gyllenstierna served as military attaché in Brussels from 1936 to 1937 and was promoted to ryttmästare in 1941. The same year, Gyllenstierna was promoted to captain in the Swedish Armoured Troops. In 1943, he was promoted to captain of the General Staff Corps, and he served as aide-de-camp to the Chief of Army, lieutenant general Ivar Holmquist from 1943 to 1944. Gyllenstierna then served as a general staff officer of the III Military Area in 1944 and as a teacher at the Royal Swedish Army Staff College from 1945 to 1948 and from 1953 to 1957. He was ryttmästare in the cavalry from 1948, was promoted to major in the General Staff Corps in 1952, and served on the Medical Board of the Swedish Armed Forces from 1952. Gyllenstierna was promoted to lieutenant colonel in 1955 and served as battalion commander in the Västerbotten Regiment from 1957 and served as commanding officer of the Swedish Battalion VI in Gaza from April to October 1959. He was promoted to colonel in Västerbotten Regiment in 1960 and served as military attaché in Paris and Berne from 1960 to 1965. From 1966 until his retirement in 1972, Gyllenstierna served as regimental commander of the Life Regiment Grenadiers. He was also a board member of the Swedish Military Sports Association from 1943 to 1944 and secretary of the Royal Patriotic Society from 1973 to 1987.
Personal life
In October 1936, he married Eva Svennilson, the daughter of Sven Nilson, a court chaplain, and Auga, in Solna Church. They had three children: Nils, Christofer and Tove. His son Christofer Gyllenstierna is a Swedish ambassador. In 1970, he married Maj Dalén, the daughter of professor Albin Dalén and Maja. He later married Else Bengtson-Muusfeldt, an artist. Gyllenstierna died on 14 May 2003 in Stockholm and was buried in Skogskyrkogården on 29 August 2003.
